1. Field of the Invention
This invention relates to improvements in the operation of known electrically augmented vacuum filters, hereinafter EAVF, in dewatering aqueous suspension. More specifically this invention relates to the use of special membranes to restrict the migration of certain ionic species and particulate solids in dewatering with EAVF. The invention is especially concerned with improvements in dewatering suspensions of kaolin clay.
2. Prior Art
Filtration of slurries by electrokinetic methods (electrophoresis, electroosmosis) has a relatively long history. Reference is made to U.S. Pat. No. 1,229,203 to Schwerin (1917) which discloses electrically dewatering dispersed suspensions of fine-particle size solids. Recent interest in electrofilters has intensified as a result of increased demand for hydrated (uncalcined) kaolin clay in the form of dispersed fluid high-solids (e.g. 70%) slurries instead of dry powders and to produce higher feed solids to clay dryers when it is necessary to ship in dry powder form. A commercially utilized design of such a unit is disclosed in U.S. Pat. No. 4,168,222 and U.S. Pat. No. 4,107,026 to Freeman, the entire disclosure of which are incorporated herein by cross-reference thereto. These patents describe the workings of an electrically augmented vacuum filter (EAVF). An electro-osmotic/phoretic process for concentrating clay is disclosed in U.S. Pat. No. 4,110,189 to Kunkle et al. An electroflocculation cell is disclosed in U.S. Pat. No. 4,048,038 to Kunkle, and an electrokinetic cell is disclosed in U.S. Pat. No. 3,980,547 also to Kunkle.
The incorporation of an electrofilter as part of an improved system for producing calcined clay by utilizing the electrofilter to dewater relatively dilute, e.g. 40% solids, slurries of dispersed kaolin to levels suitable for spray drying, e.g. 55 to 60% solids, is disclosed in the allowed application Ser. No. 1,898 to Mixon, which application is commonly assigned with applicant's. The entire disclosure of said application is incorporated herein by cross-reference.
Clay is conventionally wet-processed by slurrying crude clay in water with the aid of dispersing chemicals such as sodium silicate, sodium hexametaphosphate, sodium tripolyphosphate and tetrasodium pyrophosphate. Certain polymeric organic materials are also employed, alone or in combination with inorganic dispersants. The clay slurry is blunged, degritted, classified and bleached to remove undesirable foreign matter and to improve brightness and other end-use properties. In many operations the clay is also beneficiated to remove discrete impurities especially those that are colored. Following these steps, the clay is dewatered or filtered to produce a solid filter cake of 50 to 60% clay. This cake may then be redispersed and spray dried or otherwise dried in the flocculated state or dispersed (deflocculated) state depending upon the end use.
For various reasons it is becoming increasingly desirable for the clay producers to ship slurries containing about 70% solids. The addition of 30 to 50% spray dried clay to a slurry of 50-60% solids redispersed filter cake is conventionally practiced at the present time to yield a 70% solids slurry. This addition of spray dried clay to the filter cake adds significantly to the cost of the slurry due to the relatively high costs of preparing the spray dried clay. Therefore it would be desirable to be able to prepare 70% solids slurries without the necessity of adding spray dried clay. Similarly it would be beneficial to increase the solids content of filtered feed charged to dryers when it is desired to supply the clay in dried form.
The use of an electrofilter with vacuum may be advantageous for its ability to dewater suspensions economically. The electrofilter operates on the principle that charged particles when subject to the action of an applied (direct current) electric field will move in the direction of opposite polarity. This traveling of solid particles through a liquid due to the application of an electric field is called electrophoresis. The clay particles are normally negatively charged and move toward the anode (+) electrode. When applied to aqueous clay suspensions, electrophoresis causes suspended clay particles to deposit on a filter medium surrounding the anode. Commercial EAVF systems include an anolyte chamber associated with each anode and means for charging anolyte solution into and out of the chamber. Vacuum at the anode is used to retain anolyte and remove gaseous products of reaction. Vacuum at the cathode is used to remove filtrate liquid and gaseous products of reaction at the cathode.
Other ionic species notably sodium cations arising principally from the dispersant and the anolyte, and chloride and hydroxyl anions will also move in the electric field. Reactions at the electrodes will generate ionic species and gaseous products depending upon the electrolytes used. For example, in the EAVF the anodic compartment may be filled with an electrolyte such as sodium chloride solution. Under the action of the electric field chlorine gas is produced at the anode and sodium ions will move away from the anode and react (with an electron) at the cathode to produce sodium hydroxide and hydrogen gas. Hydroxyl anions, being negatively charged, will move away from the cathode at a rate determined by the strength of the electric field and the motion of the filtrate resulting from the applied vacuum at the cathode.
In general electrically augmented filtration of kaolin slurries is often more cost-effective than conventional means, because less energy is used to achieve an equivalent result. In addition, when the cake discharge of such a filter is saleable virtually as is without further processing, the desirability of incorporating EAVF in kaolin processing schemes becomes plain.
However several problems may arise in the operation of an EAVF. Mobility of ions in the electrtokinetic cell may give rise to undesirable anions being present in the filter cake. For example, if OH.sup.- ions are present in the filter cake, optimum dispersion of the cake becomes impossible, since the slurry is overdispersed and slurry viscosity is undesirably high as a result. The ionic contamination of the filter cake using an EAVF operated with an anolyte has resulted in excessive viscosity of the clay slurry product and an excessively low specific resistance of the clay slurry product. This has limited use of the system for filtering clay intended for paper coating or paint applications. Also, under the pressure driving forces (vacuum) fine particle size kaolin may seep through the tightly woven filter cloths presently in use and contaminate the anolyte at the anode or the filtrate at the cathode.
Thus an object of the invention is to improve the effectiveness of an EAVF and thereby to enhance and extend the utility of such filtration system.
A more specific object of the present invention is to provide means for preventing undesirable anions from contaminating the filter cake on an EAVF.
Another object of the invention is to provide means for preventing fine particle size kaolin from seeping into the anolyte.
Other objects and advantages of the invention will become evident to those skilled in the art by the following description and discussion.